Effects of Wind Velocity and Nebkha Geometry on Shadow Dune Formation

Abstract

Flow dynamics and sand deposition processes over nebkhas were investigated using computational fluid dynamics simulations, wind tunnel experiments, and field measurements. The computational fluid dynamics simulations showed that nebkha width affects both the elongation and broadening of the wind shadows. The length of the wind shadows decreased as the wind shear velocity increased, following a power function, irrespective of the width and height of the nebkha. There are some uncertainties about the effect of the aspect ratio of the nebkha on the formation of wind shadows as a result of the omission of the transport of sand in the simulations. It is suggested that the length of wind shadows is dependent on the absolute values of nebkha width and height rather than the nebkha aspect ratio. Although the numerical results were consistent with the results of the wind tunnel experiments, the wind tunnel experiments showed that the height of the nebkha had a negative effect on the elongation of the shadow dunes. The sedimentary structures revealed by ground‐penetrating radar surveys of the dunes in the Taitema Dry Lake, in the eastern Taklimakan Desert, China, showed that the formation of shadow dunes can be divided into three phases. Shadow dunes are initially controlled by horizontal separation flow and then elongate following the same growth mechanisms as linear dunes, eventually breaking up into isolated barchans or short, temporary linear dunes. However, the formation of shadow dunes does not necessarily call for the three phases depending on the local wind regime and sediment supply.

A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures

“…Some similarities are shared with terrestrial "sand shadows" of various types (Bagnold, 1941, pp. 189-191;Hesp, 1981;Hesp & Smyth, 2017;Yang et al, 2019;Zhao et al, 2019), in which a sand deposit extends downwind from a small, nonerodible Journal of Geophysical Research: Planets obstruction (commonly a clump of vegetation, or an isolated rock) in a unidirectional wind regime. The sand deposit is stable only within the aerodynamic protection provided by the upwind obstruction, and shrinks downwind to its termination for two reasons: (1) small, natural variation of wind azimuths around the dominant wind direction create a triangular protection zone that narrows downwind behind the obstacle; and (2) even without small variations of wind azimuth, the boundary layer eventually recovers to normal strength downwind of the obstacle, as demonstrated even in the perfectly unidirectional flows of numerical and wind tunnel experiments (Hesp & Smyth, 2017;Yang et al, 2019).…”

Soil Organic Carbon, Total Nitrogen and Their Densities of <i>Nitraria Tangutorum</i> Nebkhas at Different Succession Stages on the Edge of Ulan Buh Desert

“…As a momentous part of the desert ecosystem, nebkha is a kind of wind-driven sedimentary landform developed under the influence of vegetation [10], nebkha, as the linkage between local climate and environment, has the function of soil and water conservation, wind prevention and sand fixation [15]. Previous studies have focused on the distribution of nebkhas [10], its morphological characteristics [17], vegetation diversity and productivity [18], sediment characteristics and supply, and its influencing factors [19][20]. These studies have reported that the formation of nebkhas is mainly controlled by wind intensity, sand sources and vegetation coverage.…”

Aeolian sand transport thresholds in excavated foredune notches

“…During the last decade, flow–form–sand transport interactions have been studied on various coastal dune landforms, including foredunes (de Winter et al, 2020; Delgado‐Fernandez et al, 2013; Hesp & Smyth, 2016; Hilton et al, 2016; Jackson et al, 2013, 2011; Lynch, Jackson, & Cooper, 2009; Lynch, Jackson, & Cooper, 2010; Petersen, Hilton, & Wakes, 2011; Schwarz et al, 2021; Wakes, Bauer, & Mayo, 2021; Wakes, Hilton, & Konlechner, 2016; Wakes et al, 2010; Walker et al, 2017), foredune scarps (Hesp & Smyth, 2019; Piscioneri, Smyth, & Hesp, 2019), blowouts (Garès & Pease, 2015; Hesp & Walker, 2012; Hugenholtz & Wolfe, 2009; Pease & Gares, 2013; Smyth, Jackson, & Cooper, 2012; Smyth, Jackson, & Cooper, 2013; Smyth et al, 2019; Smyth, Jackson, & Cooper, 2014), parabolic dunes (Anderson & Walker, 2006; Delgado‐Fernandez et al, 2018; Hansen et al, 2009; Smyth et al, 2020; Wakes, 2013), nebkha (Hesp & Smyth, 2017; Zhao et al, 2019, 2020, 2021), sand cays (Hilton et al, 2019), beaches with large woody debris (Grilliot, Walker, & Bauer, 2018; Grilliot, Walker, & Bauer, 2019), and beach scarped ridges (Smyth & Hesp, 2015). These studies have contributed to our understanding of flow–form–sand transport relationships in a wide range of aeolian situations.…”